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Might be bit of a silly question really as there are many different calipers designed specifically around rotor size like the old hope IS mount calipers but here goes. Specifically for long travel forks 200mm is there a minimum rotor size or would it just depend on personal preference and riding style? Obviously you wouldn't use a 200mm travel fork in a daily commute bike nor would you run a 160mm rotor on a downhill beast.

  • This will depend on too many factors to answer. It will depend on the weight and the hill. VTC – paparazzo Aug 7 '17 at 17:22
  • If I want to stop I want to stop immediately. Upsizing the front rotor is not all bad, but there will be increased weight and aero effects. You're likely to hit the tyre/road adhesion limit first, leading to a slide. – Criggie Aug 7 '17 at 21:14
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Since post mount became the norm, most long travel/gravity forks have a minimum rotor size based on what post mount length standard they're built with. Here is how Fox communicates it on the product page for their 40 series forks for example:

Fox 40 specs

So putting the caliper directly on the post without any adapter sets it up for a 203, and the fork can't take anything smaller. If you had a fork with 180mm post mounts and you wanted to run a 200mm rotor, you would use a +20mm adapter, and you would not be able to run 160. If it's not specified anywhere, which is often the case, usually that means 160 front, 160 rear for mountain and hybrid/urban and 160 front, 140 rear for road.

Sometimes manufacturers make it difficult to track down the number. SRAM/Rockshox only sticks it in their giant tech spec pdfs as far as I know.

This is the answer as it applies to current forks. In the past there were more forks that needed model-specific adapters, and various IS-only calipers based on specific rotor sizes. So in the scheme of things, there are also all sorts of older forks and brakes where the minimum or maximum rotor size was limited in those ways. In the more common case of a fork with standard IS mounts, the minimum is always 160.

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  • Thanks for that. I'm struggling setting up a 2004 marzocchi 888R which is IS mount. I'm using old hope mono m4 brakes which are IS mount. It's set fine with 225mm rotor and plus 40 adapter but the rotors are very difficult to get hold of in that size so trying to setup with a smaller possibly a 200mm rotor as they are easier for me to get hold of – user31395 Aug 7 '17 at 20:35
  • I see. I believe that fork has normal IS mounts and that the IS mount Mono M4 is made for a 180mm rotor in front. If that's all true you should be able to use the Hope HBIS20 adapter for a 200mm rotor, or no adapter for a 180. – Nathan Knutson Aug 7 '17 at 21:26

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